Dual solder layer for fluidic self assembly and electrical component substrate and method employing same

ABSTRACT

A dual solder layer for fluidic self assembly, an electrical component substrate, and method employing same is described. The dual solder layer comprises a layer of a self-assembly solder disposed on a layer of a base solder which is disposed on the solder pad of an electrical component substrate. The self-assembly solder has a liquidus temperature less than a first temperature and the base solder has a solidus temperature greater than the first temperature. The self-assembly solder liquefies at the first temperature during a fluidic self assembly method to cause electrical components to adhere to the substrate. After attachment, the substrate is removed from the bath and heated so that the base solder and self-assembly solder combine to form a composite alloy which forms the final electrical solder connection between the component and the solder pad on the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional PatentApplication Ser. No. 61/678,933, filed Aug. 2, 2012 and entitled “DUALSOLDER LAYER FOR FLUIDIC SELF ASSEMBLY AND ELECTRICAL COMPONENTSUBSTRATE AND METHOD EMPLOYING SAME”, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

The placement of electronic components using self assembly is becomingan important approach for high-volume production of electronicassemblies. For example, it is well-known to use fluidic self assembly(SA) in the production of radio-frequency identification tags (RFIDs).In that approach, sub-millimeter integrated packages with distinctdimensions and trapezoidal shapes are dropped into an agitated fluidwhere they fit into specific matching depressions on a substrate.Packages that don't fall into depressions are removed and redroppeduntil all are matched. Circuit connections are then made by masking anddepositing conducting strips over the electronic packages. This approachworks well at high volumes, but requires very specificgeometrically-shaped components or packages and substrates which have tobe specially etched to accommodate the packages.

A more general approach that has been investigated does not requirespecially-shaped packages and can use more standard components. In thisapproach, components are dropped into an agitated fluid where they findproper locations on a substrate through contact and adherence usingvarious approaches. For example, hydrophilic and hydrophobic materialsmay be coated on the components and desired substrate locations, orbonding sites, such that when parts find proper locations they tend tostick when the same coatings come into contact, i.e.,hydrophilic-hydrophilic or hydrophobic-hydrophobic; mixed coatings donot stick. Agitating the fluid is also necessary since it randomized themotion of the components, allowing them to make contact attempts withall regions of the substrate. Furthermore, if they don't stick on thefirst attempt, agitation allows the components to make many attemptsuntil they finally find a bonding site.

One of the best ways to achieve the self assembly is by using the strongwetting effects of solder on a metal contact to “pull” components intoplace. Unlike other SA bonding materials, solder also has a highlubrication; this implies that once the component makes contact with thesolder, the component can find the minimum energy configuration withminimal friction. This wetting effect occurs when the solder is aliquid, therefore self assembly of components must be done above themelting point of the solder. In the case of solder SA, one immerses thesubstrate and the electronic components in a liquid, allowing the liquidto carry components into their positions. The solder wetting effecttakes over when the components come into contact with the melted solderon the substrate, pulling the components into their final position andretaining them. Note that in particular, using solder fluxes as thebinding agent is not useful for SA because of their low degree oflubrication.

For solder SA, low melting temperature solder (T_(m)<150° C., whereT_(m) is the melting point) is used for a variety of reasons. One reasonis that simple lower viscosity non-toxic fluids such as water are easyto use, but obviously require temperatures to be below their boilingpoint. Furthermore, since electronic or opto-electronic components areimmersed in the hot liquid on the order of one minute in typical SAruns, high temperatures may damage the components. Unfortunately, verylow temperature solders (T_(m)<100° C.), generally require Bi whichgenerally leads to poor bonding and therefore unreliable long termattachment of components. Solder compositions such as Sn—In can haveT_(m)=145° C., but again reliable bonding is not acceptable for longterm attachment of components. In addition, use of solders with such lowmelting points may be problematic for long term operation of componentssuch as light emitting diodes (LEDs) whose operating temperatures mayapproach or even exceed the melting point of such low temperaturesolders.

U.S. Patent Publication No. 2010/0139954 to Morris et al. discloses anapproach by which solder or fluid based-SA can be performed at practicaltemperatures while still providing a method to permanently electricalbond components with reliable higher temperature solders. The approachuses multiple sites that perform different functions. In particular, acentral site on the component is used for a SA binding site whilespatially separated sites closer to the part boundaries are used forelectrical bonding. Generally the electrical bonding sites are solderbumps. All contacts are on the bottom of the component and are designedto mate with matching sites on the substrate. The central binding siteon the substrate supports a low temperature solder (or other material)that forms a hemispherical shape when liquefied. The height of thecentral SA solder when liquid exceeds the height of the solid electricalsolder bumps. The solder bumps melt at a higher temperature than thecentral binding site solder. In the embodiment described, eutectic Bi—Snsolder (T_(m)=138° C.) is employed for SA binding sites which bind tosolder bumps on electrical components. The solder bumps are composed ofeutectic Sn—Pb (T_(m)=183° C.) which are well known to form reliable,high conductivity electrical connections. Assembly is performed in twosteps. In the first step, components and substrate are placed in afluidic bath at a temperature above the melting point of the solder ormaterial on the central SA biding site, but below that of the solderbumps. Self-assembly onto the central pads is performed in the liquidbath. When components contact the central SA solder that is on thesubstrate, the bulging profile relaxes because of the additional wettingof the component contact. The assembled substrate is cooled to fix thecomponent locations. The substrate is then placed into a reflow ovenwhere temperatures are above the melting point of the solder bumps whichmust then expand to reach the contacts on the components. While thisapproach permits electrical connections with more reliable and higherconductivity, components and substrates require additional contacts andpads which lead to greater fabrication complexity. More significantly,practical applications of this method require components with solderbumps and additional solder masks for coating only the SA binding siteswith low temperature solder. This leads to longer overall manufacturingtimes and cost, both of which SA should alleviate. Other problems occurwith this method because the physical height changes of the SA soldersand electrical bonding solders must be compatible with the process.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

The present invention solves the above problems by combining theadvantages of low temperature solder for self assembly (SA) with ahigher temperature, reflowable solder to make a final, more reliablejoint.

This invention employs a two-layer method by which fluidic solder-basedself assembly can take place with at low temperatures while finalsoldering can be done using higher temperature solders which are knownto be more reliable. Furthermore, unlike the separated electricalcontact/binding site approach described above, self assembly andelectrical connections can be made with the same contact pad. Thispermits much more flexibility in component contact and substrate padconfigurations. Additionally, because low and high temperature soldersare both applied to the same bond pad, the assembly process isstraightforward, decreasing assembly time and reducing cost.

Two layers of solder are used for each electrical contact. No othercontacts are required. Thus contacts serve both as SA binding sites andsolder connection sites. The base layer solder is suitable for highreliability and high electrical conductivity. Typically this would besolder used for reflow. The second top layer is low melting temperaturesolder or liquid metal that is used for SA. In one embodiment, this toplow temperature SA solder layer would be thinner than the base layer,the proportions being dependent on the compositions of the soldersinvolved. However, this is not a definitive constraint in the invention.This scheme would allow for the low temperature SA solder layer to meltand bond components to the substrate during the self assembly step aftercooling. Since parts are already bonded at the proper locations, thecooled substrate would be reheated to temperatures above the meltingpoint of the base solder layer either in a reflow oven or other means inwhich both layers of solder would melt. With the proper selection ofsolders, the two solder layers would mix and form a more reliable andhigher conductivity joint than would be possible with the lowtemperature SA solder alone.

In accordance with an aspect of the invention, there is provided anelectrical component substrate comprising at least one solder pad havinga dual solder layer, the dual solder layer comprising a first layer of aself-assembly solder and a second layer of a base solder, the secondlayer of the base solder being disposed on the solder pad and the firstlayer of the self-assembly solder being disposed on the second layer,wherein the self-assembly solder has a liquidus temperature less than afirst temperature and the base solder has a solidus temperature greaterthan the first temperature.

In accordance with another aspect of the invention, there is providedmethod for fluidic self assembly, comprising:

(a) obtaining an electrical component substrate, the substratecomprising at least one solder pad having a dual solder layer, the dualsolder layer comprising a first layer of a self-assembly solder and asecond layer of a base solder, the second layer of the base solder beingdisposed on the solder pad and the first layer of the self-assemblysolder being disposed on the second layer, wherein the self-assemblysolder has a liquidus temperature less than a first temperature and thebase solder has a solidus temperature greater than the firsttemperature;

(b) immersing the electrical component substrate and at least oneelectrical component in a fluid bath at the first temperature so thatthe self-assembly solder liquefies;

(c) agitating the fluid bath so that the electrical component adheres tothe liquefied self-assembly solder;

(d) removing the electrical component substrate from the fluid bath;

-   -   (e) heating the electrical component substrate to a second        temperature that is greater than the solidus temperature of the        base solder so that the base solder and self-assembly solder        combine to form a composite alloy; and    -   (f) cooling the composite alloy to form an electrical solder        connection between the electrical component and the solder pad        on the electrical component substrate.

In one embodiment, the first temperature is less than about 150° C., andmore preferably less than about 100° C.

In another embodiment, the mass ratio of the self-assembly solder to thebase solder is less than 1, and more preferably between 0.5 and 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C schematically illustrate an embodiment of a two-layer,solder-based self-assembly substrate and method according to thisinvention.

FIGS. 2A and 2B are exemplary binary phase diagrams for self-assembly(SA) solders and base solders, respectively.

FIG. 3 is a binary phase diagram for Ga—Sn.

FIG. 4 is a binary phase diagram for Ga—Zn.

FIG. 5 is a binary phase diagram for Ga—In.

FIG. 6 is a graphical representation of the calculated mass fraction ofelements in a composite solder after reflow for Sn(0.5)-Ga(0.5).

FIG. 7 is a graphical representation of the calculated mass fraction ofelements in a composite solder after reflow for Sn(0.21)-Ga(0.79).

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

An embodiment of the self-assembly method employing an embodiment of anelectrical component substrate in accordance with this invention isillustrated in FIGS. 1A-1C. For the sake of example, we consider SA ofidentical one-contact components. The two-layer solder has already beenapplied to the solder pads 16 of electrical component substrate 18. InFIG. 1A components 10 are placed in a liquid bath 20 whose temperatureis above the melting point of the upper SA solder layer 12 but below themelting point of the base solder layer 14. The upper SA solder layer 12is therefore liquid at the temperature of the bath whereas the basesolder layer 14 which is attached to solder pad 16 on substrate 18remains solid. Generally, the bath 20 consists of fluids such as wateror ethylene glycol that do not react or damage components, solders, orsubstrate materials. The fluid is normally agitated to increase theprobability of component attachment to solder sites on the substrate.The components 10 have their electrical contacts coated with gold orother noble metals to which the liquefied upper SA solder layer 12 willeffectively wet. Preferably, the components 10 are LED dies.

After some time all or most of the parts will have attached to desiredsolder binding sites and the SA process is terminated. The timenecessary to achieve the desired attachment yield is determined by theagitation rate, the physical parameters of the liquid bath, temperature,solder surface energies, component contact characteristics, and otherfactors. The substrate is then cooled to allow the SA solder tosolidify. In some embodiments, the SA solder is not required tosolidify, provided the binding strength of the liquid SA solder is highenough to prevent components from detaching when removing the substratefrom the bath. The resulting configuration for the case of solidified SAsolder is shown in FIG. 1B.

In a final step, the base solder layer 14 is melted so that components10 form electrical solder connections 22 to the solder pads 16. This canbe accomplished by a variety of methods known in the art, but typicallywould consist of passing the substrate through a reflow oven withdesired time-temperature profiles. During this final reflow period bothbase and SA solder layers melt to form a liquid whose composition nowconsists of the mixture of both solders. During cooling, the solder meltwill solidify forming electrical solder connections 22 comprised of anew composite alloy with the desired electrical and mechanicalcharacteristics. This is depicted in FIG. 1C.

Since the two solders form a composite alloy, the compositions and molarfractions of both base and SA solders should be chosen appropriately.Possible materials for the low melting point required for the SA solderpreferably include systems with low toxicity (Hg, Pb, Sb, and Cd-free),high surface energy for reliable SA, and dissolution into common Pb-freereflow solders that would be used for the base solder. Dissolution ofthe SA solder into the base solder is normally possible if the molarfraction of the SA solder is not too large; less than 10% is typical.Alternatively, one could choose the molar concentrations of the SA andbase solders such that they are on a eutectic; however, this requirescareful monitoring of SA and base solder mass ratios on each solder pad.Therefore, the eutectic method is less desirable. In a third method, theSA solder molar fraction may have values greater than 10%, perhaps 50%or more. The solidified composite forms a new alloy, albeit differentthan the original alloy of the base solder. This is the preferredmethod, because the application of the SA solder is least constrainedand can be applied by simple “tinning” methods in which the substratejust contacts a liquid SA solder bath.

In many cases, it may be desirable to have the SA bath be attemperatures even close to room temperature. This would generally limitthe SA solder to either pure elemental or metal alloys consistingprimarily of Ga, In, Bi, and Sn, since many alloys of these constituentsare known with melting points below even 100° C. Use of pure Ga orGa-alloys in particular may be particularly advantageous because ofgallium's low toxicity and very low melting point of 29.8° C. Ga is alsosoluble in other alloys. Preferably, the base solder is a solder with amelting point well above the expected operating temperature to minimizelong term failure such as creep or oxidation but well below anycomponent or substrate damage threshold.

In a first embodiment, the SA solder is chosen to have a lower meltingpoint than the base solder. No requirements are made regardingcompositions such as being eutectic, binary, or other physical factorsor properties. In particular, the self-assembly process occurs at atemperature T₁>T_(SA,L), where T_(SA,L), is the liquidus temperature ofthe SA solder. Additionally, the base solder is chosen so it's solidustemperature T_(B,S)>T₁. Furthermore, during the reflow or finalattachment process, the temperature for forming the final electricalsolder connections, T₂, is higher than the liquidus for the base solder,or T₂>T_(B,L) and will always exceed the liquidus temperature of theinitial SA solder alloy. The final criterion is that the temperature T₂also be high enough to keep the entire SA and base solder solution inthe liquid state; that is T₂>T_(comp,L), where T_(comp,L) is theliquidus temperature of the final composite alloy that forms from theliquid solution of the SA and base solders.

For the sake of illustration, FIGS. 2A and 2B show two hypotheticalphase diagrams for binary SA and base solder alloys, respectively,together with the above defined temperatures. The SA solder is assumedto be a binary alloy consisting of components A and B with compositionsranging from 0% to 100% of B, inclusive. Similarly, the base solder isassumed to consist of components C and D with compositions ranging from0% to 100% of D, inclusive. In the example, the SA and base solders areassumed to have compositions xA+(1-x)B and yC+(1-y)D, respectively,where x and y are the respective molar fractions of A and C. In the caseof the SA solder, the solid form consists of mixtures of A-rich a phaseor solution and B-rich β phase. Note that choosing eutectic compositionsfor the SA and base solders are not necessarily advantageous because thefinal composition of the composite SA and base alloys determine thefinal reliability properties of the soldered components.

In general, SA solder systems for this embodiment may consist of a puremetal, binary, ternary, or higher order alloys which use at least one ofthe following low melting point elements: Ga, In, Bi, Se, Sn, and Zn.Antimony is also a possibility in some applications, but its toxicity isgreater than Ga, In, Bi, Sn, and Zn. Alkali metals generally have verylow melting points, but their reactivity is highly undesirable forsolder. The SA solder alloys may also include small impurities (<1%) ofthe elements including, but not limited to Ag, Au, Al, Cu, Ge, Ni, orSi. Such impurities are normally not desirable for the SA solder becausethey will increase the melting point of the SA solder, but may be eithernecessary to enhance final composite solder properties or dissolutionbehavior as the two-layer system is heated to the higher reflowtemperature. Furthermore, such impurities may be unavoidable forpractical solder systems.

For the next embodiments of the invention, we consider SA soldercompositions that have as low a melting point as possible, but will notincur undesirable properties in the final composite solder joint. Thebase solder is preferably a compatible lead-free solder. To reduce thenumber of elemental choices for the SA solder, we consider the lowestmelting point elements that are usable in low toxicity solders: Ga, In,Bi, and Sn (Table 1). Note that indium generally induces creep in solderjoints and should probably be avoided in most applications. It also isexpensive. However, certain indium compositions have been shown to havegood solder properties which are therefore useful in two layer solderself-assembly.

TABLE 1 PRIMARY ELEMENTS FOR SELF-ASSEMBLY SOLDERS Element Melting point(T_(m) in ° C.) Ga 29.8 In 156.7 Sn 232 Bi 271

One important characteristic for two-layer solder self-assembly is themass ratio R_(M) of SA solder, M_(SA,) to base solder, M_(B), whereR_(M)=M_(SA)/M_(B). Generally this ratio will be much less than onebecause the SA solder normally contains substantial amounts of a lowmelting element to keep the SA solder melting point at reasonablevalues. On the other hand, these elements generally have smaller massfractions in the final composite solder in order to have reasonablesolder properties and higher melting points needed to minimize creep andother issues when components operate above ambient temperatures.Self-assembly however is better suited if the mass ratio R_(M) is closerto one-half or more. This is because enough molten material must bepresented to the components in the liquid bath to form adequate bindingsites. Furthermore, application of the SA solder would generally be donein molten form and will strongly wet the underlying solid base solder;therefore controlled application of small amounts of SA solder is moredifficult. Additionally, one would want to keep the total mass of baseand SA solders to amounts similar to conventional one-layer reflow orother solder processes. Therefore, placing large amounts of base solderon the substrate to accommodate deposition of larger amounts of SAsolder is not practical. With these concepts in mind, we considerexamples practical systems for two-layer solder self-assembly in thenext embodiments.

For a second embodiment, we consider Bi binary alloys. Bi—Sn is one ofthe best choices as it has a eutectic temperature of 139° C. at 57weight % Bi. While Bi solders have some reliability tradeoffs, includingbrittleness and moderate toxicity, they may have applications in certaintwo-layer self-assembly processes. Thus one may use Bi(58)-Sn(42)eutectic solder for the SA solder of mass M_(SA). Note all fractionalsolder alloy compositions are in weight percent (wt. %). For example,Bi(58)-Sn(42) contains 58 wt. % Bi and 42 wt. % Sn. For a compatiblebase solder one may use compatible alloys of Sn—Ag or Sn—Ag—Cu. If oneuses for example a commercially available alloy (Indium Corp.)Sn(96.5)-Ag(3.0)-Cu(0.5) (T_(L)=220° C.), the composite solder afterreflow will be close to a well known systemSn(90)-Ag(2.0)-Bi(7.5)-Cu(0.5) which has a melting point in the range of198-212° C. This is suitable for reflow bonding and is within anacceptable temperature range for components such as LEDs.

The required mass fraction, R_(M), of SA solder to base solder can befound by the formula:

$\begin{matrix}{{R_{M} = {\frac{x_{{Bi} - {SA}}}{x_{{Bi} - {Comp}}} - 1}},} & (1)\end{matrix}$

where x_(Bi-SA) is the mass concentration of Bi in the SA solder andX_(Bi-Comp) is the mass concentration in the composite solder afterreflow. For the case of 7.5% Bi in the composite solder, the massfraction of SA solder to base solder is approximately 0.15. The finalcomposite solder composition is Sn(89.4%)-Ag(2.6)-Bi(7.5)-Cu(0.4) whichis not too far from the desired composition.

In general, one would make trade-offs between higher mass fractions ofSA solder which may be beneficial for deposition of SA solder andself-assembly requirements, while lower mass fractions reduce Bi contentand may give better composite solder properties. For example, a solderSn(91.8)-Ag(3.4)-Bi(4.8) has shown excellent long-term electrical andthermal properties with a melting point in the range of 200-216° C. Inthis case one could start with a base solder of Sn(96.5)-Ag(3.5)eutectic alloy (available from Indium Corp.) which has a melting pointof 221° C., which is somewhat high but acceptable for reflow. With adesired mass fraction R_(M)=0.090, the resulting composite solder isvery close to the desired Sn(92.0)-Ag(3.2)-Bi(4.8).

In general, one can use a eutectic Sn—Bi alloy for the SA solder,together with a variety of available Sn—Ag alloys for the base solder,with Sn mass concentrations of at least 80% and preferably greater than90% to achieve usable reflow temperatures. Additionally, variations onthe eutectic Sn—Bi alloy such as ternary alloys Sn(42)-Bi(57)-Ag(1.0)have melting temperatures (T₁=140° C.) very close the binary eutectic,but may have additional desired properties. Such ternary alloys havebeen used in commercial products.

In a third embodiment, we consider alloys with In, which may give goodsolder properties for the final composite solder alloy. Quaternaryalloys of the form Sn—Ag—Bi—In, have been investigated wherein the Inmass concentration varied from 2.5-8.0%. In particular, the higher Incomposition Sn(88)-Ag(3.5)-Bi(0.5)-In(8) was found to have good strengthand joint reliability in long-term testing, while having a reasonablesolidus melting point T_(s)=165° C. and a liquidus point of T_(L)=206°C. Because In has a low melting point (Table 1), it is very useful forSA solder and can permit a more balanced mass ratio of SA solder to basesolder than Bi-based SA solders.

A good choice for an In-based SA solder is the eutecticIn(50.9)-Sn(49.1) alloy which has a melting point T_(m)=120° C.,somewhat lower than the Bi—Sn eutectic. From Equation (1) for the caseof In, we find that the mass fraction R_(M)=0.195 and the desired basesolder composition is Sn(96.2)-Ag(4.2)-Bi(0.6). The liquidus point forthis ternary alloy is about 235° C. which is slightly higher thandesirable due to the formation of a Ag₃Sn phase. Reducing the Agconcentration in the base solder to 3.5% gives a better liquidus pointof about 220° C. Using the reduced Ag concentration and altering thebalance of Sn in the base solder gives a final composite solderSn(88.6)-Ag(2.9)-Bi(0.5)-In(8) which should have properties close to thesolder described above. Note that more generally, a variety of In—Bi—Snand In—Bi alloys have even lower melting points the In—Sn eutectic andmay be useful for SA solders with the proper base solder composition.

In a fourth embodiment, we consider Ga alloys for the SA solder becauseof gallium's very low melting point (Table 1) and desirablemetallurgical properties when used as an additive with other solderalloys. The very low Ga melting point implies a wide range of Ga alloyswill also have low melting points over a large composition range,permitting self-assembly at temperatures closer to room temperature. Asdiscussed this is strongly desirable for two-layer solder self-assembly.Binary alloys of Ga include Ga—Sn, Ga—Zn, and Ga—In. Phase diagrams forthese alloys are shown in FIGS. 3-5, respectively.

For the sake of example with Ga, we consider the simplest SA solder andbase solder compositions. Referring to FIG. 3, we consider a base soldercomposition of Sn(1-y)-Ga(y), where y is the mass fraction of Ga. If themaximum usable SA solder liquidus were assumed to be below 150° C., thenthe fraction of Ga, y>0.25. For more reasonable temperatures ofT_(L)=100° C. or 50° C., y=0.5 and 0.79 respectively. A reasonable basesolder is the eutectic solder Sn(96.5)-Ag(3.5) with a melting point ofT_(m)=221° C., well above the liquidus temperatures for the range ofSn(1-y)-Ga(y) alloys considered. FIG. 6 shows calculated mass fractionsof the composite alloy as a function of R_(M), the SA solder to basesolder mass ratio assuming an SA solder composition ofSn(0.50)-Ga(0.50). A similar calculation in shown in FIG. 7 for thelower temperature SA solder composition of Sn(0.21)-Ga(0.79).

In principle, a wide range of binary, ternary, and even quaternaryGa-based SA solder alloys may exist, including Ga with major componentsof Sn, Zn, In, and Bi. For the base solder, lead-free alloys may bebased on binary systems that include Sn such as Sn—Ag, Sn—Au, Sn—Zn,Sn—Cu, Sn—Bi, and Sn—In. Suitable alloys may further include lesseramounts of Cu, Al, Ni, and Mg to aid solder properties. However, SAsolder and base solder compositions are not limited to thesecombinations.

While there have been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims.

What is claimed is:
 1. An electrical component substrate, comprising atleast one solder pad having a dual solder layer, the dual solder layercomprising a first layer of a self-assembly solder and a second layer ofa base solder, the second layer of the base solder being disposed on thesolder pad and the first layer of the self-assembly solder beingdisposed on the second layer, wherein the self-assembly solder has aliquidus temperature less than a first temperature and the base solderhas a solidus temperature greater than the first temperature.
 2. Theelectrical component substrate of claim 1, wherein the first temperatureis less than about 150° C.
 3. The electrical component substrate ofclaim 1, wherein the first temperature is less than about 100° C.
 4. Theelectrical component substrate of claim 1, wherein a thickness of thefirst layer is less than a thickness of the second layer.
 5. Theelectrical component substrate of claim 1 wherein a molar fraction ofthe self-assembly solder in the dual solder layer is less than about10%.
 6. The electrical component substrate of claim 1 wherein a massratio of the self-assembly solder to the base solder is less than
 1. 7.The electrical component substrate of claim 1 wherein a mass ratio ofthe self-assembly solder to the base solder is between 0.5 and
 1. 8. Theelectrical component substrate of claim 1 wherein the self-assemblysolder is gallium or an alloy of gallium.
 9. The electrical componentsubstrate of claim 1 wherein the self-assembly solder and the basesolder contain a same low melting point element selected from Bi, In andSn.
 10. The electrical component substrate of claim 1 wherein theself-assembly solder and the base solder contain Bi, and a massfraction, R_(M), of the self-assembly solder to the base solder isdetermined by a formula${R_{M} = {\frac{x_{{Bi} - {SA}}}{x_{{Bi} - {Comp}}} - 1}},$ whereinx_(Bi-SA) is a mass concentration of Bi in the self-assembly solder andX_(Bi-Comp) is a mass concentration of Bi in a composite solder formedby combining the self-assembly solder and the base solder.
 11. Theelectrical component substrate of claim 1 wherein a molar fraction ofthe self-assembly solder in the dual solder layer at least 10%.
 12. Theelectrical component substrate of claim 1 wherein a molar fraction ofthe self-assembly solder in the dual solder layer is at least 50%. 13.The electrical component substrate of claim 1 wherein a melting point ofthe base solder is above an operating temperature of an electricalcomponent to be mounted to the electrical substrate and below atemperature at which the electrical component or substrate is damaged.14. The electrical component substrate of claim 1 wherein theself-assembly solder contains at least one of a low melting pointelement selected from Ga, In, Bi, Se, Sn and Zn.
 15. A method forfluidic self assembly, comprising: (a) obtaining an electrical componentsubstrate, the substrate comprising at least one solder pad having adual solder layer, the dual solder layer comprising a first layer of aself-assembly solder and a second layer of a base solder, the secondlayer of the base solder being disposed on the solder pad and the firstlayer of the self-assembly solder being disposed on the second layer,wherein the self-assembly solder has a liquidus temperature less than afirst temperature and the base solder has a solidus temperature greaterthan the first temperature; (b) immersing the electrical componentsubstrate and at least one electrical component in a fluid bath at thefirst temperature so that the self-assembly solder liquefies; (c)agitating the fluid bath so that the electrical component adheres to theliquefied self-assembly solder; (d) removing the electrical componentsubstrate from the fluid bath; (e) heating the electrical componentsubstrate to a second temperature that is greater than the solidustemperature of the base solder so that the base solder and self-assemblysolder combine to form a composite alloy; and (f) cooling the compositealloy to form an electrical solder connection between the electricalcomponent and the solder pad on the electrical component substrate. 16.The method of claim 15, wherein the self-assembly solder is solidifiedprior to removing the electrical component substrate from the fluidbath.
 17. The method of claim 15, wherein the first temperature is lessthan about 150° C.
 18. The method of claim 15, wherein the firsttemperature is less than about 100° C.
 19. The method of claim 15wherein a melting point of the base solder is above an operatingtemperature of the electrical component and below a temperature at whichthe electrical component or substrate is damaged.
 20. The method ofclaim 15 wherein a mass ratio of the self-assembly solder to the basesolder is less than 1.